ASHRAE Journal- September 2008

Toplighting & Lighting Controls For Commercial Buildings By Tyson Lawrence; Kurt Roth, Ph.D., Associate Member ASHRAE; Drury B. Crawley, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE L ighting accounts for about 20% of all commercial building energy consumption, more than any other end use.1 Daylighting, i.e., using lighting controls to dim or turn off electric lights when photosensors detect sufficient levels of natural light indoors, has the potential to produce large reductions in commercial building lighting energy consumption. When properly implemented, daylighting can reduce lighting energy use by 40% to 60%.2 Some building owners, researchers, and toplighting practitioners believe that daylighting may provide other benefits, e.g., increased occupant health and well-being, that could increase productivity or sales. 3 At present, the portion of floor space with adequate fenestration for effective daylighting limits the ultimate savings potential of this approach. Estimates suggest that windows (side lighting) can currently effectively daylight approximately 15% to 20% of commercial building floorspace.4 Existing toplighting installations, primarily skylights, but also monitors and sawtooths, can only effectively daylight approximately 2% to 5% of commercial floorspace.5 Challenges associated with daylighting include:3 • The cost of windows/skylights and lighting control systems; • Added design complexity to ensure siting and interior space facilitates daylighting; • Glare (large intensity variations) management; and • Implementation of effective lighting controls. Often, toplighting eases some of these challenges relative to sidelighting. Notably, glare is easier to reduce, and lighting control can be simpler and more reliable due to more consistent light distribution throughout the day. Furthermore, toplighting has the potential to provide greater energy savings per unit area, because it can provide up to 30% more light per square foot of glazing.2 Energy Savings Potential Toplighting integrated with photosensor-based lighting controls affects building energy consumption in several ways. First, it directly reduces lighting energy consumption by dimming or turning off electric lights during periods of the day when daylight can meet some or all of indoor lighting needs. Second, toplighting reduces cooling energy consumption by reducing the heat dissipated by electric lighting. Third, the increased solar gain of toplighting relative to an opaque roof increases cooling energy consumption during the cooling season while decreasing heating energy consumption during the heating season. Fourth, the reduced thermal resistance of toplighting relative to a conventional roof (e.g., U-values of 0.8 versus 0.06 Btu/ hr•°F•ft2)3 increases cooling loads (and energy consumption) when the outdoor air temperature exceeds the indoor temperature, increases heating loads when the outdoor air temperature is (approximately) lower than the balance temperature for the building, and decreases cooling loads between the two aforementioned regimes. As shown in Figure 1, reductions in lighting energy consumption have a much greater energy cost—and energy consumption—impact than any of the other factors noted at economically optimum skylight-to-floor area ratios (SFRs*). However, the detrimental impact on heating and cooling energy cost and energy consumption can become significant at appreciably higher SFRs.3 A recently completed study found that an economically optimum deployment of skylights in several different building types and climates could reduce annual lighting energy consumption by 35% to 55%. Stated another way, depending on climate and building type and using national average commercial sector utility rates, toplighting can reduce commercial building energy costs by $0.12 to $0.32/ft2. The national primary energy savings technical potential equals about 0.4 quads, assuming complete deployment for all floor space located directly below a roof in offices, educational buildings, big-box retail buildings, and warehouses.3 Market Factors Despite the large energy savings potential of toplighting with lighting controls, initial estimates indicate that only approximately 2% to 5%5 of commercial floor space currently has sufficient skylight area for toplighting, compared to 60% of commercial floor space that is directly under a roof and, there- * Ranging from 2% to 4% of building floor space, depending on building type and climate. 92 ASHRAE Journal ashrae.org September 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal- September 2008

ASHRAE Journal- September 2008 Section: Contents Contents Section: Commentary Options for Sustainability Section: Industry News The Silk Route for Energy Solar Thermal Is Unrealized Opportunity Industry Groups Sue City of Albuquerque Section: Letters Letters Section: Meetings and Shows Meetings and Shows Section: Feature Articles Article-Radiant Floor Cooling Systems Article-HVAC Design for Sustainable Lab Article-Mixed Mode Ventilation Article-Single- Design Considerations For Active Chilled Beams Article-Acoustic Design In Green Buildings Article-Teams, Contracts & BIM Section: Building Sciences Some Old Lessons Distilled Section: Washington Report Energy in Federal Buildings Section: Products HVAC&R Product Showplace Section: Emerging Technologies Toplighting & Lighting Controls For Commercial Buildings Section: Special Products Fans & Blowers Section: Classified Ads Classified Ads Section: Advertising Index Advertising Index

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